Roof top ice dams destroy millions of dollars worth of property annually. As snow on your roof melts, ice dams form at the roof edges and gutters (also known as icicles). As more snow melts, water gets trapped behind the ice dams. As the trapped water grows it begins moving upward inside the roof shingles and leaks into the house. The short term effect is mold. The longer term effects include, structural damage, ceiling damage, household goods damage, premature roof failure, and flood damage.
We had to use ice melt socks to open channels in the roof ice dam so water could drain off the roof instead of dripping into the ceiling.